The present invention relates to a subscriber line interface circuit for a telephone system, and more particularly to a current or voltage subscriber line interface circuit in which the interface circuit is isolated from high AC voltages, such as from a ringer generator without a relay or other device which might substantially distort the signal on the telephone line.
A telephone system subscriber line interface circuit (SLIC) connects a balanced two-wire transmission path (the path to and from the subscriber telephone handset) with an unbalanced four-wire transmission path (the path between telephone central offices). SLICs perform various functions, including battery feed, overvoltage protection, ringing, signaling, hybrid, and timing.
SLIC operation is known and need not be considered in detail. By way of brief explanatory example, and with reference to FIG. 1, a current mode SLIC 10 (a SLIC that processes currents related to the signals in the transmission paths, rather than the voltages) uses current information available in the tip and ring voltage sensing resistors R1 and R2 at the two-wire side of the SLIC. The current information is provided to a hybrid circuit 12 that detects incoming signals and sends them in the correct direction. The hybrid circuit 12, in combination with external circuitry 14 connected to the four-wire side of the SLIC, provides appropriate amplification, transhybrid echo cancellation, and impedance matching.
The present invention is related to the application of ringer generator signals to the telephone circuit. With reference to FIG. 2, a SLIC 30 may be connected to a telephone instrument (represented by the resistor 32) through tip and ring paths (T, R). When the tip and ring paths are carrying a voice signal, the signal is generally on the order of 3 volts in amplitude, requiring current on the order of 4 ma in amplitude, having a bandwidth of up to 16 kHz, and a precision (or distortion) on the order of tenths of a percent. The voice transmission signal is bidirectional and sometimes the battery feed is superimposed on the voice signal, yielding a current of up to 30 ma in amplitude. Moreover, the tip and ring paths (and all switches contained therein) cannot introduce any crossover distortion when the current through the circuit changes direction.
Generally, telephone instruments are caused to "ring" by the instrument's detection of a ringer signal across the tip and ring leads. The ringer signal is selectively applied to the transmission path to alert the telephone instrument of an incoming call and generally remains on the transmission path until the telephone is "answered" by taking the telephone instrument off hook. While the tip and ring paths must carry both the voice and the ringer signals (not necessarily simultaneously), the characteristics of the two types of signals are markedly different. For example, a typical ringer signal may have voltage amplitudes of plus-to-minus 150 volts, current amplitudes of up to 200 ma, a bandwidth of 75 Hz. Generally, distortion and crossover distortion of the ringer signal are not important considerations for ringer signals.
Combining the requirements of the two types of signals, voice transmission and ringer signals, yields the requirement that the circuit between the SLIC and the telephone instrument must function as a two channel multiplexer (or switch), where one switch must pass a voice signal without affecting the bandwidth or distortion level and the other switch must pass the very high currents of the ringer generator and must withstand and block the very high ringer voltages when the switch is off. With reference to FIG. 2, switches SW1 and SW2 may be selectively operated to apply the ringer signal, generated by the ringer generator 34, to the transmission path. At or before the time that the ringer signal is applied to the transmission path, the SLIC is protected by opening the circuit between the ringer generator and the SLIC using the switches SW3 and SW4.
This requirement has been met in the prior art by relays. As is well known, the relay is a short circuit when on and an open circuit when off. Generally, a relay has the characteristics of an ideal switch, simultaneously meeting the precision and bandwidth requirements of the voice switch and the current and voltage requirements of the ringer switch. Electro-mechanical relays are well known in the prior art but are comparatively expensive, slow, occupy valuable circuit real estate, and consume relatively high amounts of power to operate. Accordingly, it is known in the prior art to use solid state switches and multiplexers; however, such devices are generally far from ideal and cannot fully satisfy all the requirements simultaneously. Attempts have been made in the prior art to integrate the required solid state switches in series with the hybrid of the SLIC; however, such attempts have usually resulted in a substantial degradation of the voice signals.
Accordingly, it is an object of the present invention to provide a novel method and apparatus for coupling a SLIC and a ringer signal to a telephone instrument.
It is another object of the present invention to provide a novel method and apparatus for isolating a SLIC from the ringer signal.
It is yet another object of the present invention to provide a novel method and apparatus for integrating with a SLIC a means for isolating the SLIC from a ringer signal.
It is still another object of the present invention to provide a novel method and apparatus for interfacing between a SLIC and a ringer signal without substantial degradation of the voice signal.
It is a further object of the present invention to provide a novel method and apparatus in which the means for isolating a SLIC from other portions of a telephone circuit are integrated into the same integrated circuit as the SLIC.
It is still a further object of the present invention to provide a novel method and apparatus for isolating a SLIC from a ringer signal without placing a switch between the tip and ring terminals of the SLIC and the ringer generator.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention applies from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.